1. Field of the Invention
This invention relates to a method of fabrication of a light image detector and to a two-dimensional matrix detector which is obtained by means of said method in the general field of fabrication of electronic circuits in thin films on large surfaces.
The application contemplated in the present invention is the integrated control of each elemental point of a two-dimensional matrix image detector.
2. Description of the Prior Art
At the present time, the principal applications in which the high photoconductivity of amorphous silicon is associated with the possibility of fabricating electronic control circuitry from the same material and on the same substrate are concerned with linear image detectors of large size (up to 20 cm) which are necessary for reading documents corresponding to the international page format A4 by lateral displacement of the document with respect to the sensor.
Three different structures have been contemplated for the construction of linear detectors of this type:
the association of a photoconductor, of a storage capacitor, and of a thin-film transistor (TFT) of amorphous silicon (a-Si) as described in the article by M. Matsumura in the review entitled "IEEE Electron Device Letters", vol. EDL - 1, No. 9, September 1980, pages 182 to 184; PA0 the association of a photodiode of amorphous silicon and of a TFT of amorphous or polycrystalline silicon as described in "Extended Abstracts of the 15th Conference on Solid State Devices and Materials", Tokyo (1983), pages 201 to 204, by F. Okumura; PA0 the association of a photodiode and of a blocking diode of a-Si as described by Y. Yamamoto in "Extended Abstracts of the 15th Conference on Solid State Devices and Materials", Tokyo (1983), pages 205 to 208. PA0 Length: 50 mm PA0 Resolution: 8 to 10 bits/mm PA0 Size of pixel: 100 ?m.times.70 ?m PA0 Reading time: 2 ?s/bit without multiplexing. PA0 on a substrate covered with a first layer of conductive material, a matrix of photodiodes in the form of pads arranged in rows and columns and each comprising a layer of amorphous semiconductor material doped with a predetermined type, a layer of undoped amorphous semiconductor material, a layer of doped amorphous semiconductor material doped with another predetermined type, a second layer of conductive material, each photodiode being insulated from adjacent photodiodes by means of insulating material; PA0 on the insulating material, columns deposited along the columns of photodiodes and each formed by a layer of metallic material and a layer of doped amorphous semiconductor material; PA0 connection elements each connected to a photodiode through the layer of conductive material of the photodiode, located in proximity to a column and each formed by a layer of metallic material and a layer of doped amorphous semiconductor material; PA0 rows disposed along the rows of photodiodes and overlapping the columns as well as at least one connection element at each point of intersection of a row and a column, each row being formed by a layer of undoped amorphous semiconductor material, an insulating layer and a layer of metallic material.
A certain number of these structures make it possible to obtain devices having the following characteristics:
On the basis of these results, it would appear reasonable to expect the forthcoming construction of linear sensors which permit reproduction of documents under good conditions. By way of example, it will be feasible to reproduce documents having a width of 216 mm at a rate of 8 bits per millimeter and at a paper transfer rate of 5 ms per line.
A two-dimensional image detector having a matrix addressing and reading system has also been developed such as the detector described in the article by S. Uya in Extended Abstracts of the 16th Conference on Solid State Devices and Materials, Kobe, (1984), pages 325 to 328. The image detector described in this article is formed of a matrix of photodiodes of amorphous silicon superposed on a two-dimensional array of charge-coupled devices (CCDs) for reading the charges created by light. Since the CCD is formed of monocrystalline silicon, the size of the sensor is limited by requirements which are specific to the monocrystalline silicon technology, namely a prototype of less than 1 cm on each side.
In contrast, the present invention relates to a large-sized two-dimensional image detector with matrix addressing and reading.
The elemental point is constituted by an amorphous silicon photodiode connected to the drain of an amorphous silicon control transistor. The source of said transistor is formed by a column electrode and the gate is controlled by a row electrode. If so required, the second terminal of the photodiode which is common to all the diodes can be at a nonzero potential in order to reverse-bias the diode.
The present invention benefits by the progress achieved both in the field of photovoltaic diodes and in the recent performances obtained in the fabrication of control transistors for addressing liquid-crystal display screens of the flat panel type.
One of the principal advantages of the method in accordance with the invention lies in the fact that it requires only three masking levels. Furthermore, it permits the construction of large-area detectors of much greater size than those in current use.
By way of example, one advantageous application of said detector could be the construction of a two-dimensional x-ray detector on condition that a scintillator which absorbs x-rays and re-emits visible rays is interposed between the x-ray source and the detector. This would be one way to replace the photosensitive plates or the x-ray image intensifiers at present employed in radiology.